Influence of Species, Age and Diet on Mercury Concentrations in Shetland Seabirds

MARINE ECOLOGY PROGRESS SERIES Published May 22 Mar Ecol Prog Ser I

Influence of species, age and diet on mercury concentrations in Shetland seabirds

F. M. Stewart, R.A. Phillips*, P. Catry, R. W. Furness

Applied Ornithology Unit, IBLS, University of Glasgow. Glasgow G12 800, United Kingdom

ABSTRACT:Chick down, chick feathers and feathers from adults of 5 seabird species (Arcticskua Ster- corarjus parasiticus, great skua Cdtharacta skua, Arctic tern Sterna paradjsaea, kittiwake Rissa tridactyla, and common guillemot Uria aalge) were analysed for mercury. Individual female Arct~cand great skuas' body feather mercury concentrations correlated wlth concentrations in their chicks' down, but not feathers (Arctic skua. r = 0.64; great skua: r = 0.66) This demonstrated that mercury in chick down originated from the egg, and that mercury in the egg and In adult females' plumage could have the same dietary source. Inter-specific differences in mercury concentrations were found for all age classes sampled, and these could be explained partly in terms of dietary specialisation, although physiological variations may also be important. All 3 age classes of great skua showed a direct incl-easein mercury with increasing proportion of bird meat in the diet of individual pairs. In kittiwake, Arctic skua and great skua, adults had higher mercury concentrations thanchicks and ch~ckdown had higher con- centratlons than chick feathers. However, In 2 species (Arctic terns and guillemots) chick down had higher concentrations than adult feathers. Chick dnlvn could be sampled for mercury content ds an alternative to using eggs in national biomonitoring xhernes. Feathered chicks could be sa~iipledto determine mercury availability around the breedlng colony between hatching and fledging.

KEYWORDS: Bionionltorlng Diet. Heavy metals Individualvar~at~on . hlercury Seablrds

INTRODUCTION temporal studies and geograph~cal comparisons (Walsh 1990, Thompson et al. 1992).However, when Seabird tissues have been widely used as biomoni- interpreting data on mercury concentrations in feath- tors of heavy metals in the marine environment. Signif- ers, several factors must be considered. Birds are com- icant temporal and geographical variations have been plex organisms and physiological and environmental demonstrated,polluted environments have been iden- factors will affect mercury burdens. tified and contamination patterns monitored (Becker Two major determinants influencing mercury con- 1989, Walsh 1990,Thompson et al. 1992, Becker et al. centrations in seabirds are diet and age. Inter-specific 1993~1,b). differences In mercury burdens are often attributed to Feathers are often used in such analyses, and as variations in diet or trophic level (Hutton 1981, Ohlen- monitoring units they have several unique advantages. dorf & Harrison 1986, Braune 1987, Becker et al. 1994, A sample of body f~dthersprovides a consistent and Wenzel & Gabrielsen 1995), but studies attempting to reliable measure of the total mercury burden of quantify this are lacking.Age has no effect on mercury the bird (Furness et al. 1986), sampling is relatively concentrations in feathers once birds are fully grown straightforward and non-invasive, and there is a (Furnesset al. 1990,Thompson et al. 1991,Burger et al. wealth of material in museums which can be used for 1994); mercury accumulated in internal tissues since the end of the previous moult cycleis excreted into the 'Present address: Dept of Biological Sciences, University of new plumage and thus gives a measure of mercury Durham, Durham DH1 3LE,United Kingdom. accumulated in the inter-moult period (Furness et al. E-mall. r.a.phillips@durh~r~~,ac.uk 1986). By contrast, mercury concentrations In the

0 Inter-Research 1997 Resale of fullarticle not permitted 238 Mar Ecol Prog Ser 151 237-244, 1997

plumage of chicks and immature birds may be gradu- dynamics as the plumage changes from down to feath- ally incorporated into down and feather during growth. ers. Finally, we look at intra-specific variation in indi- The mercury concentrations in chick down are thought vidual mercury burdens in relation to diet of great to reflect concentrations in the egg (Becker & Sperves- skuas. lage 1989, Becker et al. 1993a), but there are very few studies which have demonstrated a relationship between mercury concentrations of breeding females MATERIALS AND METHODS and that of the egg they produce (Lewis et al. 1993, Burger & Gochfeld 1996). Sample collection. Fieldwork took place in June- Most studies have found that mercury concentra- July 1994 on. Foula (60" 08' N, 2'05' W), a small island tions in adults were higher than in chicks or sub-adults approximately 22 km west of Shetland mainland. (Furness et al. 1990, Thompson et al. 1991. Monteiro et Feathers were taken from samples of adults of 3 spe- al. 1994, Stewart et al. 1994, Wenzel & Gabrielson cies (common guillemot, Arctic tern and kittiwake), 1995).However work on common terns Sterna hirundo down and feathers from Arctic tern and kittiwake has demonstrated higher concentrations of mercury in chicks, and down only from guillemot chicks during feathers (Monteiro & Furness 1995) and very similar the routine yearly nnging programme. Six to ten body concentrations in the liver and feathers of chicks com- feathers were removed from each adult and feathered pared to adults (Gochfeld & Burger 1987, Burger et al. chick and stored in polythene bags. Samples of down 1994) were plucked from the flank area of each chick. All Studies which have investigated the age-related vari- birds were ringed, weighed and measured (maximum ation within chicks have come to apparently contra- wing chord). dictory conclusions. Mercury concentrations in chick Adult Arctic skuas and great skuas were nest- feathers showed increases with age in eastern great trapped during incubation using a clap net, and white egrets Egretta alba modesta and common terns feather samples taken as above. Arctic skua and great (Honda et al. 1986, Becker et al. 1993a),but were inde- skua nests were marked and breeding performance pendent of age in great skuas Catharactaskua (Thomp- was monitored for other ecological studies. Great son et al. 1991) and negatively correlated with age in skuas were sexed by observation. Arctic skuas were several species from the Azores (Monteiro et al. 1994). sexed by observation, by discriminant analyses where Analysis of the plumage of chicks can be more infor- the probabilities of group membership were >0.85, or mative than that of adults in some circumstances, and by association with a partner sexed by one of these many researchers have advocated chicks as useful methods (Phillips & Furness in press). Chicks from long-term biomonitors (Walsh 1990, Furness 1993, marked nests were sampled twice, first as downy Monteiro & Furness 1995).Chicks accumulate mercury chicks and again when their feathers had grown. Not over a short period and can therefore provide informa- all great skua chicks were sampled a second time as tion on bioavailability of metals from a 1oca.lised forag- several disappeared due to predation and fieldwork ing area and over a specific time period (from hatching ended before some were old enough to have grown to fledging). This however can be complicated by the feathers. residual mercury component in chicks derived from The diets of great skua adults were determined by the adult female via the egg, which may be excreted weekly collection and analysis of regurgitated pellets into chick down (Becker et al. 1993a). This bias could from marked territones. These contain the indigestible be minimised by choosing chtck feathers rather than components of prey items such as fish otoliths, feath- down for analysis, depending on the requirements of ers, mammalian fur, goose barnacle Lepas sp. plates the monitoring programme. etc. Pellets are considered to be representative of prey This study compares mercury concentrations in taken by the adults, and indicative of food fed to chicks feathers of adults and chicks (downy and feathered at the nest (Furness & Hislop 1981). The proportion of young) of 5 species of sea.bird from Shetland, UK (corn- all pellets sampled that consisted partly or wholly of mon guillemot Uria aalye, Arct~ctern Sterna paradis- feathers was calculated in order to determine the rela- aea, Arctic skua Stercorarius parasiticus, kittiwake tive importance of bird flesh in the dlet of individual Rissa tridactyla, and great skua), to investigate the pairs. For every territory marked and monitored for effects of species, age and diet on mercury concentra- diet, at least downy chicks were sampled for mercury tions. Individually marked Arctic skuas and great concentrations. skuas were sampled to look at the relationship Chick ages. Chick ages were calculated from wing between mercury concentrations of individual adults length for common guillemots (Harris et a1 1991) and and those of their chicks. Chicks of all species Arctic terns (Ewins 1985), and from body mass for kit- were sampled at different ages to determine mercury tiwakes (Galbraith 1983). Chick ages for Arctic and Stewart et al.. Mercury concentrations in seab~rds 239

great skuas were calculated from observed hatching correlated with chick age in guillemot down (r = -0.42, date. p = 0.023, n = 29), and in kittiwake down (r = -0.60, p = Mercury analysis. Total mercury concentrations 0.040, n = 12),and feather (r = -0.45, p = 0.020, n = 26). were determined by a cold vapour technique using a There were no significant relationships with age in Data Acquisition Ltd DA 1500-DP6 hilercury Vapour Arctic skua chick down (r = -0.25, p = 0.134, n = 36) or Detector, preceded by a standard acid digestion (Fur- feather (r = 0.14, p = 0.451, n = 30), nor in Arctic tern ness et al. 1986). Feathers were dried to ambient labo- down (r = 0.04, p = 0.858, n = 24) or feather (r = 0.32, p = ratory temperature (ca 22°C) before analysis. The 0.246, n = 15). Similarly, mercury concentrations in reproducibility and accuracy of the mercury determi- great skua chicks did not show significant relation- nation technique were tested by analysing Interna- ships with age in down (r = -0.06, p = 0.624, n = 58), tional Atomic Energy Agency Horse Kidney Reference although there was a negative, but just non-signifi- Material H-8 (Thompson & Furness 1989). All mercury cant, trend in feather (r = -0.35. p = 0.066, n = 28). concentrations are expressed as pg g-' on a dry weight basis. Statistical analysis. All statistical analyses were car- Inter-specific differences ried out using the SPSS-PC+ package (Norusis 1986, 1988) and Zar (1984). Inter-speciflc differences in mercury concentrations were found in all age classes. There were significant differences in mercury concentrations between adults RESULTS of the 5 seabird species (Kruskal-Wallis ANOVA, x2= 118.05, p 0.0001, n = 160). Non-parametric ranges Mercury concentrations for all species and age classes test (Zar 1984) indicated that the great skua adults had are shown in Table 1 In general, adults showed a significantly higher concentrations than all other spe- greater variation In mercury concentrations than either cies, kittiwake adults had significantly higher concen- feathered chicks or downy chicks, as shown by their trations than both Arctic terns and guillemots, but not higher coefficients of variation. The exception to this is Arctic skuas, and Arctic skuas had h~gherconcentra- the Arctic tern where downy chicks showed slightly tions than Arctic terns and gu~llemots.There was no greater inter-individual variation compared to adults. difference between the last 2 species (Fig. 1). Inter-specific comparison of mercury concentrations in feathered chicks showed a different pattern. There Chick age and mercury concentrations were differences in mercury concentrations between the species (Kruskal-Wallis ANOVA, y,' = 68.22, p < Analysis of the effects of age on mercury concentra- 0.0001, n = 99). Non-parametric ranges tests showed tions were perforined separately for downy and feath- that great skua concentrations were significantly ered chicks. Mercury concentrations were negatively higher than all others and Arctic tern chicks had

Table 1 Mercury concentrations (pgg.') in body feathers of adults and feathers and down of chicks. SD: standard deviation. SE: standard error, CV: coefficient of variation

Species-- Age class Mercury mean (n) SD Guillemot Adults Uria alalge Downy chicks Kittiwake Adillts Rjssa tridactyla Downy ch~cks Feathered chicks Arctic tern Adults Sterna paradjsaea Downy c-hlcks Feathered chlcks Arctlc skua Adults Stercorarlus parasllic~~s Downy chicks Feathered chicks Great skua Adults Cdtharacta skua Downy chicks Feathered ch~cks Mar Ecol Prog Ser 151. 237-244, 1997

species. There were no differences between Arctic terns, Arctic skuas and kittiwakes, but both Arctic terns and Arctic skuas had higher concentrations than guillemots. There were no differences between guillemots and kittiwakes (Fig. 3).

Intra-specific age class differences

All species showed significant differences In the pattern of variation between age classes in mercury concentrations. Guillemot chicks had significantly higher mercury concentrations than adults (t-test, t = -3.04, df = 61, p = 0.004). There were significant differences between mercury concentrations with age class in kittiwakes (Kruskall- Wallis ANOVA, xZ= 48.26, p < 0.0001, n = 59). There were no differences between adult feather and chick down concentrations, but both were higher than chick Fig. 1 Inter-specific d~fferencesin adult feather mercury lev- feather concentrations. els for 5 species of seabird. Error bars are SD. (0) Species In Arctic terns there were significant differences in between which there were no differences in mercury levels. All other differences were significant mercury concentrations between the age classes (Kruskall-Wallis ANOVA, x2= 44.70, p < 0.0001. n = 62). There were no differences between adults and higher concentrations than kittiwakes and Arctic feathered chicks, but downy chicks had higher con- skuas. There were no differences between kittiwakes centrations than either of these groups. and Arctic skua chick feather concentrations (Fig. 2). For Arctic skuas, again there were age class differ- Mercury concentrations in downy chicks also ences (Kruskall-Wallis ANOVA, x2= 57.61, p < 0.0001, showed significant inter-specific differences (Kruskal- n = 94). There was no difference between concentra- Wallis ANOVA, = 114.88, pc 0.0001, n = 159).Great tions in adult and downy chicks, although both were skua chicks had higher concentrations than all other higher than in feathered chicks.

Species

Fig. 2. Inter-specific differences in chick feather mercury lev- Fig. 3 Inter-specific differences in chick down mercury Ie\-els els for 4 species of seabird. Error bars are SD. (01 Species for 5 species of seabird. Error bars are SD. (0) Spec-res between which there were no differences in mercury levels. between which there were no differences in mercury levels. All other differences were significant All other d~fferenceswere significant Stewart et al.: Mercury

For great skuas there were differences between age feathers (r = 0.75, p < 0.001, n = 20) and chick down classes (Kruskall-Wallis ANOVA, x2= 83.16, p < (r = 0.36, p < 0.01, n = 56). 0.0001, n = 140). Adults had higher concentrations than either chick age class, and chick down concentrations were higher than chick feather concentrations. DISCUSSION

Adults and their chicks Comparison between individuals and their broods in Arctic skuas and great skuas Mercury concentrations in the down of chicks is thought to originate from the egg, with mercury being A clear relationship was found between adults and transported into the down during embryonic develop- their chicks in both Arctic and great skuas. The con- ment. Becker & Sperveslage (1989) found that concen- centrations of mercury in the adult females' body trations of mercury in eggs and 5 d old downy chicks of feathers correlated with their chicks' down (Arctic herring gull Larus argentatus from the same clutch skua: r = 0.64, n = 20, p = 0.002; great skua: r = 0.66, p = were strongly correlated, and concluded that the mer- 0.002, n = 20) but not feather mercury concentrations cury found in chick down was due mainly to the con- (Arctic skua: r = 0.13, p = 0.599, n = 19; great skua: r = centrations in the egg. 0.23, p = 0.492, n = ll), whereas the mercury concen- Mercury concentrations in eggs are thought to re- trations in the male birds did not show any relationship flect local contamination of mercury in food items in- with chick down (Arctic skua: r = -0.05, p = 0.837, n = gested in the period just prior to egg-laying (Barrett et 16; great skua: r = -0.080, p = 0.816, n = 11) or feather al. 1985, Becker 1992, Becker et al. 1993b, Furness (Arctic skua: r = 0.36, p = 0.249, n = 12; great skua: r = 1993, Lewis et al. 1993, Monteiro & Furness 1995). 0.74, p = 0.156, n = 5). Lewis et al. (1993) measured mercury concentrations Paired comparisons indicate that individual chicks in feathers, eggs and soft tissues of herring gulls. They had significantly higher concentrations of mercury in found that mercury concentrations in eggs were not down than in their feathers in both Arctic skuas (paired correlated with concentrations in feathers of individual sample t-test, t = 12.91, df = 28, p < 0.001) and great females, although they were signif~cantlyrelated to the skuas (palred sample t-test, t = 9.89, df = l?, p c 0.001). liver concentrations. The implication from these results However, there were no correlations between mercury was that mercury in feathers and in the liver did not concentrations in down and feathers of individual Arc- have the same source; feather mercury would origi- tic skua (r = 0.237, p = 0.217, n = 29). or great skua nate from food eaten during the moulting period plus chicks (r = 0.20, p = 0.428, n = 18). the mercury stored in soft tissues between moults (Fur- Mercury concentrations in Arctic skua chicks from ness et al. 1986). Therefore, mercury concentrations in the same brood were correlated in down (r = 0.58, p = the eggs were thought to originate from mercury in- 0.04, n = 12), but not in feather (r = -0.06, p = 0.85, n = gested immediately prior to egg-lay~ngand thus reflect 11). Great skuas showed a similar pattern: mercury local contamination at the breeding site. Becker et al. concentrations in chicks from the same brood were (1993b) showed high inter-site variations in mercury correlated for down (r = 0.82, p = 0.013, n = 8). There levels in both eggs and chick feathers which indicated was not a large enough sample to test this for concen- differences in local contamination. However, our re- tration in chick feathers. sults suggest that this may not always be the case. The significant positive relationship between mercury concentration in plumage of adult female great and Arctic Diet and mercury concentrations skuas and the down of their chicks indicates a close re- lationshlp between mercury concentrations in the The proportion of bird flesh in great skua diet (as adult plumage and egg mercury concentrations. In ad- indicated by pellet analysis) showed considerable vari- dition, there was no correlation between mercury con- ation, with some pairs apparently feeding entirely on centrations in the down of individual chicks and that in discarded gadid fish obtained from demersal fisheries, their feathers, which would be expected if mercury in contrast to others which appeared to consume only concentrations in each resulted from accumulation bird flesh. The dietary data were converted to the pro- from prey caught close to the colony. As chick feather portion of all pellets that contained feathers, and data mercury concentrations were clearly much lower than were arcsine transformed. Great skua mercury con- down concentrations, it may be hypothesised that ei- centrations in plumage were significantly and posi- ther mercury found in the egg is accumulated over a tively correlated with the proportion of bird meat in the longer time period by the female prior to laying, adults diet for adult feathers (r = 0.37, p 0.02, n = 40), chick accumulate more mercury than chicks due to higher Mar Ecol Prog Ser 151: 237-244, 1997

food intakes (especially around laying), egg mercury was found in adult feathers (in the case of terns more does contain a portion of the female's body burden ac- than double; adult mean = 0.86 pg g.', chick down

cumulated since the previous moult, or that individual mean = 2.03 pg g l).This was also recorded in common females have year-round dietary preferences which terns breeding in the Azores where hatchlings had a consistently influence mercury inputs into eggs and mean of 4.8 pg g.' mercury In their down compared plumage. If it were true that adults accumulate more with 2.5 yg g-' in the feathers of adults (Monteiro & mercury than chicks because of a higher food intake Furness 1995). Both Arctic terns and guillemots within a short time period, then one would still not ex- undergo a partial pre-nuptial moult (Ginn & Melville pect a correlation between adult plumage and chick 1983) which would reduce the body burden of mer- down concentrations, as is seen here. The fact that we cury. In the female, tissue mercury levels are further have also found a relationship between summer diet reduced by excretion into the eggs and consequently and mercury levels in feathers of great skuas is also im- body feathers grown in the post-nuptial period will portant. Levels of mercury In body feathers are have lower levels related to this reduced body pool. thought to reflect year-round accumulation, but it may Adult females may lower their body pool of mercury be that summer d.iet has a greater influence on mer- between moults by excreting a significant amount of cury levels in feathers than previously realised. A posi- what is a potentially toxic material into the egg tive correlation was also found between mercury levels (Braune & Gaskin 1987, Becker 1993a, Lewis et al. in body feathers of females and eggs of Franklin's gull 1993). Indeed, Lewis et al. (1993) estimated that in Laruspipixran (Burger & Gochfeld 1996) This species comparison with males, female herring gulls could returns to the colony only a few days before egg-laying potentially excrete over 20% more mercury via their and therefore the source of mercury must originate eggs. Other studies on auks and gulls also found sig- from outside the breeding area. nificantly lower mercury concentrations in feathers of females than males (Braune & Gaskin 1987, Stewart et al. 1994). Age-related trends

Kittiwakes, Arctic skuas and great skuas demon- Inter-specific differences strated similar age-related trends in mercury concentrations: adults had a higher concentration in their There were significant differences in mercury con- feathers than was found in chick down (although the centrations between species in all age classes. Such difference was only significant in great skuas), which inter-specific variations will result from differences in was in turn greater than levels in chick feathers. Adults diet, body size, moult strategy, migration patterns, would be expected to have accumulated greater con- physiology, or a combination of these (Walsh 1990, centrations because of a long exposure time to dietary Monteiro & Furness 1995). Frequently diet is consid- mercury since the previous moult (Furness et al. 1990, ered to be the most important factor (Hutton 1981. Thompson et al. 1991, Monteiro et al. 1994, Wenzel & Ohlendorf & Harrison 1986, Braune 1987, Becker et al. Gabrielson 1995). Ch~ckdown concentrations were 1994, Wenzel & Gabrielsen 1995). In this study higher than chick feather concentrations as mercury in mercury concentrations accumulated by feathered down originates from the adult female via the egg (see chicks were presumably dependent on food ingested previous section). Chick feather concentrations reflect between hatching and feather growth, and so we can the amount of mercury accumulated from around the relate the vanations in mercury concentrations be- colony (and hence circulating in the blood at the time tween species to differences in diets. The diet of great of feather formation) during the period from hatching skua chicks is much more varied than that of the other to feather growth (Furness et al. 1986, Braune & species as it may consist of sandeels, bird meat, goose Gaskin 1987, Walsh 1990) barnacles or discarded gadid fish obtained from The red.uction In mercury concentrations wlth in- trawlers, whereas the other species studied all feed creasing chick age in guillemots and kittiwakes almost exclusively on sandeels (Furness 1990, Bailey et (within the downy or feathered chick age categories) is al. 1991) The diet of great skuas would be expected to most likely due to a dilution effect. Young chicks have contain a higher mercury content as methylmercury a high rate of protein synthesis, presumably resulting shows bioamplification and many of their prey species in mercury dilution in the tissues during the phase of are from higher trophic levels. Indeed, mercurJ- con- rapid growth (Thompson et al. 1991, Monteiro et al. centrations in great skua chick feathers were clearly 1994). higher than in the other species (see Fig. 2).Kittiwake, Arctic tern and guillemot chicks had significantly Arctic tern and Arctic skua chicks are fed on approxi- higher concentrations of mercury in their down than mately the same age class and size range of sandeels Stewart et al.: Mercury concentrations ~n seabirds 243

(Furness 1990, Bailey et al. 1991) and we may expect expected as the latter 2 also reflect dietary mercury the mercury concentrations accumulated in their feath- uptake outside of the short breeding season (see previ- ers to be roughly comparable, unless factors other than ous section). These relationships indicate firstly that d~etwere involved. In fact, the pattern of mercury dietary variation has a direct and measurable effect on accumulation did not follo\v these expectations (see mercury concentrations within a species, and secondly Fig. 2) and therefore it must be concluded that physio- that chick feather concentrations are the most appro- logical differences do play an important role. In partic- priate indicator of the bioavailability of mercury during ular it seems that terns have an unusual and distinct the breeding season. pattern of mercury accumulation (Gochfeld & Burger 1987, Burger et al. 1994, Monteiro & Furness 1995, this study). Therefore, this suggests that invoking dietary Implications for biomonitoring differences to be the principle factor influencing interspecific variation in metal concentrations in chicks may Eggs have been used successfully as a monitoring in many cases be overly simplistic, and other sources of tissue to investigate geographical variation in mer- variation must be considered (Walsh 1990, blonteiro et cury and organochlorine concentrations (Walsh 1990), al. 1994). Likewise any explanation of interspecific dif- and indeed are collected as part of several national ferences in adult feather levels (Fig. 3) would require a biomonitoring schemes (Furness 1993). However, as detailed analysis of all the factors involved. Future these results show, sampling down or feathers from work should investigate the influence of these other young chicks in order to monitor mercury levels factors which contribute to patterns of heavy metal would be equally appropriate and would have several accumulation. obvious advantages. Chicks could be sampled easily without killing them, sample sizes could be much larger, and it also would eliminate either brood reduc- Intra-specific variation in diet tion or the energetic cost to the female of laying a replacement egg. Additionally, down and feather Variation in mercury concentrations within a species samples are easier to store for future analysis, as eggs can be more readily attributed to dietary variation. The have to be kept frozen. The problem still remains that relative proportion of bird meat in the diet of great we do not know whether egg and down concentra- skuas from individual territor~eswas positively corre- tions are an adequate reflection of local environmen- lated with the mercury burden of ad.ults, and both tal contamination. As our findings suggest, this may chick down and chick feather concentrations. Such a not always be the case and further work is necessary clear relationship between diet and mercury levels has to clarify the situation. never been demonstrated previously in a single free- This study has shown that chick feathers could be living species. In an earlier study, Thompson et al. sampled to measure the bioavailability of mercury to (1991) also investigated the diet of great skuas using birds in the period from hatching to fledging, although pellet analysis, but found no relationship with plumage inter-specific differences should be considered. Our mercury concentrations in adults, hence they con- results indicate that chick feathers could be an excel- cluded that winter diet must be more important in lent indicator of mercury availability around seabird determining the mercury burden. However, their study colonies particularly if coupled with dietary data. This was undertaken in a year when there were few could be useful for ongoing monitoring or geographi- sandeels available, and consequently the range of prey cal studies, especially in inshore areas most likely to be items in the diet would have been more limited. In affected by pollution (Walsh 1990).Interspecific differ- addition, there had been a recent switch to increased ences in physiology are important though and should predation on bird flesh In the absence of sandeels be ~nvestigated.The relationship between diet and (Hamer et al. 1991) and adults may have been feedin.g mercury burdens in the plumage of adult birds may be on quite different prey in the previous year. This would less straightforward because of time spent away from serve to uncouple the relationship between diet and the breeding colony during the majority of the year. adult mercury concentrations as a change in mercury intake would not be apparent in mercury burdens of plumage until the following year Acknowledgements. This research \\'as supported by a It is clear from the data that the relationship between Carnegie Trust small project grant, SOTEAG. The Nuffield diet and mercury concentrations explains most varia- Foundation and the Natural Environment Research Council. P.C. was funded by Junta Nacional de Investigaqao Cientifica 0.56), tion in chick feathers (r2 = and much less varia- e Tecnologica (Grant BD/2556/93).Thanks to Dr B. Calvo for tion in both chick down (r2 = 0.13) and adult feather help with the collect~onof feathers, and Dr D. R. Thompson mercury concentrations (r2 = 0 14). This ~~ouldbe for helpful comments on the manuscript. Mar Ecol Prog Ser 151: 237-244, 1997

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This article was submitted to the editor Manuscript first received: July 25, 1996 Revised verslon accepted: January 7, 1997